The present invention relates to a highly sensitive electron tube for quantitatively measuring an extremely weak light.
This field of technology is described in Japanese Patent Publication No. HEI-7-95434, for example. An electron tube described in this publication has a package, in which a charge coupled device (CCD) of a back-illuminated type is provided. In this type of electron tube, electron emitted from a photocathode in response to an incidence of light is directed into a back side of a device formation surface to detect a signal. This electron tube is widely used because of its high sensitivity and its high imaging quality.
An imaging device employing a back-illuminated type semiconductor device is described in Japanese unexamined patent application publication No. HEI-6-29506. The semiconductor device is fixed on a substrate whose thermal expansion coefficient is equal to that of the semiconductor device. A plurality of metal bumps are formed on the semiconductor device, each bump being connected to a metal wiring formed on the substrate (silicon wafer). The space between the semiconductor device and the substrate is filled with a nonconductive resin to prevent silicon etchant from entering therein. Since the space is filled prior to thinning of the semiconductor device, the resin has to not include alkali metal, has to have a suitable contraction stress during curing to maintain sufficient contact of the bonding part of the bumps, and has to be able to withstand heat up to about 150xc2x0 C. during die-bonding and wire-bonding.
However, conventional electron tubes and imaging devices have the following problems due to the construction described above.
In the electron tube described in Japanese patent publication No. HEI-7-95434, the semiconductor device is fixed to the stem by bonding metal pads to contacts. However, the metal pads have a tendency to slip off the contacts to lose a sufficient connection when the electron tube is assembled under a high-temperature environment.
In the imaging device described in Japanese unexamined patent application publication No. HEI-6-29506, the semiconductor device is thinned with etchant after the semiconductor device is fixed to the substrate. Accordingly, the space between the semiconductor device and the substrate is completely filled with resin in order to prevent any etchant from entering therein. Since the resin is attached directly on the electron incidence part of the semiconductor device, stress is generated in the electron incidence part when the resin cures or hardens. The electron incidence part runs the risk of becoming deformed, resulting in poor images, or in some cases broken.
In view of the foregoing, it is an object of the present invention to solve the above-described problems and to provide an electron tube which is capable of avoiding poor connections that can occur during the assembly process, as well as deformation or damage to the.semiconductor device that can also occur during this process.
These objects and others will be attained by an electron tube, comprising: a side tube; a faceplate provided at one end of the side tube and having a photocathode that emits electrons in response to incident light; a stem provided at the other end of the side tube, the stem and the faceplate defining a vacuum region, the stem having a bump connection portion on its surface; and a semiconductor device fixed to the stem at its vacuum side, the semiconductor device having a front surface positioned on the stem side and a back surface positioned on the faceplate side, the semiconductor device including an electron incidence part, for receiving electrons emitted from the photocathode, and a periphery part provided at an outer periphery of the electron incidence part, the electron incidence part having a thin plate shape whose thickness is smaller than that of the periphery part, the periphery part having a bump which protrudes from the front surface thereof the bump being fixed to the bump connection portion, the bump forming a space between the front surface of the semiconductor device and the surface of the stem, a filling material with insulation property being filled partially in the space at the periphery part, thereby partially closing the space at the periphery part.
Hence, the electron tube of the present invention includes: a side tube; a faceplate provided at one end of the side tube and having a photocathode that emits electrons in response to incident light; a stem provided at the other end of the side tube, the stem and the faceplate defining a vacuum region; and a semiconductor device fixed to the evacuated side of the stem and having an electron incidence part for receiving electrons emitted from the photocathode. The semiconductor device is configured as a back-illuminated type semiconductor device. That is, the semiconductor device has a front surface positioned on the stem side and a back surface positioned on the faceplate side. The semiconductor device has a plate-shaped electron incidence part that is formed thinner than the periphery part which is formed around the electron incidence part. A bump is formed to protrude from the front surface of the periphery part. The bump is fixed to a bump connection portion provided on the surface of the stem. The bump forms a space between the front surface of the semiconductor device and the surface of the stem. The space at the periphery part is partially filled with a filling material with insulating properties. Accordingly, the space at the periphery part is partially closed with the filling material having insulating properties.
Accordingly, in the electron tube of the present invention, insulating filling material is filled partially in the space between the periphery part of the semiconductor device and the stem, while the bump formed on the semiconductor device is connected to the bump connection portion provided on the surface of the stem. Hence, the filling material functions as a reinforcing member to prevent the bump from separating from the bump connection portion even when the electron tube is assembled under a high-temperature environment.
The space defined at the periphery of the semiconductor device is filled with insulating filling material, while the space defined at the electron incidence part is not filled with the insulating filling material. Accordingly, there is no danger of the electron incidence part becoming deformed or damaged due to stress generated when the insulating filling material is hardened.
Further, ventilation between the semiconductor device and the stem is ensured because the space between the semiconductor device and stem is only partially closed by the filling material. If the entire circumference of the periphery part of the semiconductor device were completely closed by the filling material, an air reservoir would be formed between the electron incidence part and the surface of the stem. During the process of assembling the electron tube in a vacuum, this air would expand and could cause damage to the electron incidence part which is formed as a thin plate on the back-illuminated semiconductor device. Contrarily, the present invention enables air to flow between the semiconductor device and the stem, ensuring that air can be evacuated in the vacuum environment when the electron tube is assembled.
Thus, according to the present invention, the bump protruding from the front surface of the periphery part of the semiconductor device is fixed to the bump connection portion which is provided on the surface of the stem. This bump forms the space between the front surface of the semiconductor device and the surface of the stem. The space along the periphery part of the semiconductor device is partially filled with a filling material with insulation properties. Accordingly, the space is closed only partially with the insulating filling material. As a result, it is possible to prevent poor bump connection that can possibly arise when the electron tube is assembled and to prevent damage to the semiconductor device that can occur during the same process.
The filling material with insulation property may preferably be filled in the space at the periphery part of the semiconductor device except for at least one position along the entire circumference of the periphery part, thereby allowing the space at the periphery part to be filled with the filling material with insulation property except for the at least one position.
For example, the filling material with insulation property may preferably be filled in the space at at least one position along the entire circumference of the periphery part of the semiconductor device, with a ventilating region being formed in at least one position along the entire circumference of the periphery part of the semiconductor device to provide fluid communication between the space and the vacuum region. With this construction, it is possible to avoid, by the insulating filling material, poor bump connection which can be caused when the electron tube is assembled, and to eliminate damage to the semiconductor device that can occur during the same process by ensuring ventilation through the ventilating region.
The filling material may have an electrically insulating material. The filling material may have a melting characteristic, but when heated, the filling material may be hardened and contract at an appropriate contraction stress to adhere to a surrounding material. An insulating resin is preferable as the insulating filling material. However, water glass or low-melting glass can be used.
Additionally, the stem may preferably have a supporting substrate on its surface, the supporting substrate being formed of the same silicon material as a base material of the semiconductor device, the bump connection portion being provided on the supporting substrate. With this configuration, the thermal expansion coefficient of the supporting substrate which has the bump connection portion can be.made approximately equal to that of the semiconductor device which has the bump. Therefore, the bump will not separate from the bump connection portion during the baking (heating) process in the electron tube manufacturing process, thereby maintaining a better connection state.
The bump may preferably be made of material that includes gold as a primary component. When the bump is made of material whose primary component is gold, the bump does not melt during the baking process in the manufacturing process. Further, because the insulating material, which is filled partially in the space between the periphery part of the semiconductor device and the stem, serves as a reinforcing material, the insulating material can prevent breakage in the bump, whose main component is gold, during the baking process.
The stem may have, at its surface, a channel for controlling the partial filling of the filling material with insulating property into the space at the periphery part. With this configuration, it is possible to allow an excess insulating filling material to flow into the channel when the insulating filling material is introduced from outside the periphery part into the space between the periphery part of the semiconductor device and the stem. Therefore, it is possible to prevent the insulating filling material from being attached to the electron incidence part of the semiconductor device, eliminating the possibility of the electron incidence part becoming damaged when the filling material cures or hardens. Accordingly, the filling of the insulating material can be attained appropriately without precisely controlling the amount of the insulating filling material. Especially, when the space between the semiconductor device and the stem is extremely narrow, the capillary effect can be used to force the insulating filling material to flow into the space. The excess insulating filling material automatically flows into the channel. Accordingly, control of the flow can be made easy and efficient.
For example, the channel may preferably have a width that allows the channel to span across a border between the periphery part and the electron incidence part. When the channel, whose width has a value to allow the channel to span across the border between the periphery part and the electron incident part, is formed on the surface of the stem, in order to fill the insulating filling material into the space between the periphery part of the semiconductor device and the stem, it is possible to introduce the filling material from outside the periphery part while letting the excess filling material to flow into the channel. It is therefore possible to easily prevent the filling material from attaching the electron incidence part. Especially when the space is extremely narrow, the capillary effect can be employed to draw the filling material into the space, making the process for introducing the filling material easy and efficient. When the width of the channel is set at a size to span across the border between the periphery part and the electron incidence part, several channels can be formed individually in correspondence with several regions to be filled with the filling material.
The channel may preferably be formed at a region that faces the periphery part only. When the channel is formed on the surface of the stem to confront only the periphery part, in order to fill the insulating filling material in the space between the periphery part of the semiconductor device and the stem, it is possible to introduce the filling material into the space from outside the periphery part while allowing an excess filling material to flow into the channel. It is therefore possible to easily prevent the filling material from attaching the electron incidence part. Especially when the space is extremely narrow, the capillary effect can be employed to draw the filling material into the space, making the process for introducing the filling material easy and efficient. In addition, the initial objective can be attained simply by forming the channel to correspond only to the periphery part.
The channel may preferably have a width that allows the channel to span across one side portion of the periphery part and the other opposing side portion of the periphery part. When the channel, whose width can allow the channel to span across one side portion of the periphery part and the other opposing side portion of the periphery part, is formed on the surface of the stem, in order to fill the insulating filling material in the space between the periphery part of the semiconductor device and the stem, it is possible to introduce the filling material into the space from outside the periphery part while allowing an excess filling material to flow into the channel. It is therefore possible to easily prevent the filling material from attaching the electron incidence part. Especially when the space is extremely narrow, the capillary effect can be employed to draw the filling material into the space, making the process for introducing the filling material easy and efficient. In addition, when the width of the channel is set to span across one side portion of the periphery part and the other opposing side portion in this way, it is possible to form a channel that corresponds to the size and shape of the electron incidence part of the semiconductor device.
The space formed by the bump may preferably have, at the periphery part, a height small enough to allow the filling material with insulation property to generate a capillary effect.when the filling material is drawn into the periphery part, the channel having a depth of an amount that is capable of stopping the filling material that flows due to the capillary effect. With this construction, when the filling material flowing according to the capillary effect reaches the edge of the channel, the filling material does not enter the channel but collects along the edge due to surface tension in the material. Therefore, the filling material can be easily drawn into the space and, at the same time, can be easily and effectively prevented from attaching the electron incidence part of the semiconductor device.